Methods of encrypting and using information to protect classified information are widely available. Typically, information is encrypted or decrypted using an encryption key. For example, Patent Literature 1 discloses a system for managing document use. In this system, when an electronic document stored in a document management server is edited at a client terminal, only difference data is transmitted to the document management server to update the document. Patent Literature 2 discloses a collaborative file update system for encrypting a difference file and transmitting it to an access server.
The system in Patent Literature 2 retains security by encrypting a difference file using an encryption key. Typically, an encryption key is protected with a login password in most cases. In such cases, if the password is leaked, guessed, or the like, the encryption key can be retrieved even when any strong encryption has been performed. Thus, downloading classified information is allowed on the condition that data communication with a time limit or at established intervals is carried out with a server; if the condition is not met, the classified information is automatically deleted.
For example, Patent Literature 3 discloses a content management system for deleting a duplicate copied file when a deletion request is received from a client side or when the availability period of the copied file of the duplicated content has expired.